In order to efficiently produce hydrocarbons from a subterranean formation, the formation must be sufficiently conductive in order to allow the hydrocarbons to flow from the formation to the wellbore. Various treatments for increasing the conductivity of a subterranean formation have been developed.
One technique for increasing the conductivity of a subterranean formation and thereby stimulating production of hydrocarbons from the formation is hydraulic fracturing. Hydraulic fracturing generally involves pumping one or more fracturing fluids into the formation at a sufficient hydraulic pressure to create or enhance one or more fractures in the formation. Typically, a pad fracturing fluid (“a pad fluid”) that does not contain conventional proppant particulates is first injected into the formation to initially fracture the formation. Thereafter, a slurry of proppant particulates (a “proppant slurry”) is injected into the formation. The proppant slurry places the proppant particulates in the fracture in order to prevent the fracture from fully closing once the hydraulic pressure created by the fluid is released and the fracturing operation is complete. The resulting propped fracture provides one or more conductive channels through which fluids in the formation can flow from the formation to the wellbore.
Fracturing tight or low permeability formations such as shale, sandstone and coal bed formations requires special considerations. For example, shale, sandstone and coal bed formations can each have a permeability as low as approximately one millidarcy (mD) or less. Hydraulically fracturing such formations typically forms a complex fracture network in a zone of the formation surrounding the wellbore that includes primary fractures and microfractures.
For example, microfractures can extend outwardly from the tip and edges of primary fractures in a branching tree-like manner. The microfractures can extend transversely to the trajectory of the primary fractures allowing them to reach and link natural fractures both in and adjacent to the trajectory of the primary fractures. The microfractures can exist and be formed in both near-wellbore and far-field regions of the zone. As a result, the microfractures can significantly increase contact areas with the formation matrix to give more depth and breadth to the fracture network resulting in increased production of hydrocarbons when the well is produced.
In the absence of proppant particulates, the microfractures tend to close back or seal when the hydraulic pressure placed on the formation dissipates after injection of the fracturing fluid into the well is ceased. Due to their size, conventional proppant particulates cannot be easily placed in microfractures to keep the microfractures open. Allowing the microfractures to close or seal can potentially cut off a significant portion of the fracture network and ultimately prevents the production of valuable hydrocarbons therefrom.
In order to address this issue, micro-proppant particulates having a size sufficient to allow the particulates to be placed in microfractures have been developed. For example, including micro-proppant particulates in the pad fracturing fluid places the micro-proppant particulates in the fissure openings to and otherwise in the microfractures once they are opened or created. By propping the microfractures open, the micro-proppant particulates help maintain fluid communication between the microfractures and the primary fractures and wellbore.
An additional issue that can arise when fracturing low permeability formations is removal of the fracturing fluid from the fracture network. Fluid loss to the formation can inhibit the flow of hydrocarbons through the formation during the production stage. Also, shale and clays therein can be very sensitive to water. Water imbibition by shale can cause the shale to swell and slough, and can cause clay minerals in the shale to migrate. Shale swelling and clay migration into the propped fractures can block passageways to the wellbore and cause a loss in the permeability of the formation. Also, microfractures can lose their integrity and cave in due to stresses created during the production stage.